The principal ores of molybdenum are those containing molybdenum disulfide or molybdenite. These ores commonly contain only a few tenths to one half of one percent of the molybdenite. Rhenium sulfide is associated with molybdenum in the ores. In order to recover the valuable molybdenum it is necessary to mine large amounts of the host rock, crush and grind the rock to very small particles, float the molybdenum to separate it from the host rock gangue and to separate if from contaminating minerals, and finally to roast the molybdenite to molybdenum oxide or otherwise convert it to useful chemical compounds.
The various processes involved use prodigious amounts of energy, and because of the low grades of materials being treated, result in high losses of the valuable molybdenum and rhenium.
Molybdenite once recovered is converted to molybdenum oxide by roasting it in the presence of oxygen which may be supplied by air. In the course of the roasting a great deal of heat is generated because of the exothermic reactions producing molybdenum trioxide and sulfur dioxide. Additional or excess air over that required for oxidation is frequently used during the roasting to cool the burning mass to prevent it from becoming so hot that it fuses, or that the molybdenum oxide volatilizes. As a result, the sulfur dioxide coming from the molybdenum roaster is frequently quite dilute and converted into sulfuric acid only with excessive expense.
During the roasting of molybdenite, rhenium is oxidized and at the temperature of roasting becomes volatile. Given the large volumes of gases emitted by burning organic feeds to initiate the roasting, the large volumes of sulfur dioxide from the roasting and the large volumes of air needed to cool the reaction, the recovery of the valuable rhenium is difficult and expensive.
In roasting of molybdenite, the molybdenum oxide being formed tends to coat the as yet unreacted molybenite and form an insulating layer which inhibits further reaction. As the heat and oxygen necessary to reaction must pass through this layer, it is difficult to obtain a complete roasting of the molybdenite.
In the flotation of molybdenite it is necessary to grind the ore sufficiently so as to liberate the grains of molybdenite. In doing this, often a portion of the molybdenite, which is very soft, is overground and becomes so fine (minus 10 microns) that it no longer responds well to flotation and tends to become lost in the tailings from the flotation cleaning circuits. This molybdenite could be recovered by cyclones, but its grade would be too low for practical conversion to molybdenum oxide. That is, the energy required to heat up the total gangue to roasting temperature would be too expensive to justify the added molybdenum recovery.
The grinding of molybdenite ore in order to achieve liberation of the molybdenite particles requires prodigious amounts of energy. A means of recovering the molybdenum and rehnium values without the necessity of such grinding would be a great energy and cost saver.
Microwaves are well known for their use in radar and communication transmission. They have been extensively used as a source of energy for cooking foods. Although they have been studied for many years and put to practical uses, the effects which they have on many materials is not known. The effects of microwaves upon ores and minerals is not known, nor can it be readily predicted. The effect of microwaves upon metal values contained within ores and minerals does not appear to be related in any simple way to the chemical and physical properties of such metal values. Thus, while molybdenite and rhenium sulfide strongly absorb microwaves and become heated, molybdenum oxide is largely transparent to microwaves. Copper sulfides strongly absorb, while zinc sulfide is transparent.
In treating molybdenite ores it has been found possible in accordance with this invention to selectively heat the strongly absorbing molybdenite without heating the surrounding transparent gangue silicates.
U.S. Pat. No. 2,733,983 to Daubenspeck teaches the use of ferric chloride at high temperatures of 600.degree. C. to 700.degree. C. to chlorinate nickel and cobalt oxides. U.S. Pat. No. 4,144,056 to Kruesi discloses heating a metal oxide or silicate in the absence of air with ferric chloride and a volatility depressant salt selected from the group consisting of alkali metal chlorides and ammonium chlorides for a time of about 30 minutes to about 1 hour at temperatures of from about 200.degree. C. to about 600.degree. C. Conventional heat sources are used in both processes where heat is required.
U.S. Pat. Nos. 4,123,230 and 4,148,614, both to Kirkbride, disclose the desulfurization of coal by subjecting the coal or slurry of coal particles in a hydrogen atmosphere to microwave energy to form hydrogen sulfide which is removed from the coal with solvents. U.S. Pat. No. 4,152,120 to Zavitsanos, et al, removes pyrite and organic sulfur from coal by mixing alkali metals or alkaline earth compounds with the coal and using microwave energy to selectively heat these compounds and the sulfur to convert organic and pyritic sulfur to soluble alkali and alkaline earth compounds which are removed from the coal. The subject matter of this patent is also disclosed in an article entitled "Coal Desulfurization Using Microwave Energy," Zavitsanus et al, published in U.S. Department of Commerce PB285-880, June, 1978. This patent and the article teach the use of microwave energy to selectively heat pyritic and organic sulfur contained in the organic host material coal in the presence of other elements or compounds to convert the sulfur into soluble compounds which can be readily removed from the coal. They do not teach the use of microwave energy to selectively heat metal compounds in their inorganic mineral-like host materials, alone or in the presence of other elements or compounds, to form soluble compounds of the metals which are readily recoverable from the host materials. Particularly, they do not teach the unpredictable finding that the process will work on certain ores or minerals containing metal values and not on other ores and minerals to recover their metal values.